1. Field of the Invention
Generally, this invention regards an fiber-optic, wideband array antenna beamformer and more specifically a fiber-optic, wideband array antenna beamformer using cascaded, chirped fiber gratings in a distributed architecture.
2. Description of the Related Prior Art
A large variety of current military and commercial array antenna systems require wide instantaneous bandwidths enabled through the use of a time-steered beamformer. Due to the lack of a feasible microwave alternative, much research has gone into the use of optical and photonic techniques for control of time-steered antennas. There have been numerous proposals and attempts to develop true time-delay capability optical beamformers. However, most of these techniques have not progressed beyond conceptual laboratory demonstrations, as they are hampered by the demands for precisely matched optical elements, excessive power losses, instability, or specialized component development. One of the most successful techniques for time-steered optical beamforming is the dispersive prism technique developed by Frankel et al. at the Naval Research Laboratory. See, Frankel et al.; TRUE TIME-DELAY FIBER-OPTIC CONTROL OF AN ARRAY TRANSMITTER/RECEIVER WITH MULTIBEAM CAPABILITY; IEEE Trans. Microwave Theory Techn.; Vol. 43; No. 9; pp. 2387-2394; September 1997. Although successful, this technique has some notable drawbacks directly stemming from the use of long lengths of high dispersion fiber. The long fiber lengths resulted in a system with environmental and temperature sensitivity and instability, a significant signal latency through the beamformer and a physically large system.
A number of beamforming architectures based on the substitution of fiber Bragg gratings for a high dispersion fiber have been implemented. There are the discrete fiber grating beamformer, a serially fed discrete fiber grating beam former, and a chirped fiber grating beamformer.
In the discrete fiber grating beamformer, as described by Zmuda et al., PHOTONIC BEAMFORMER FOR PHASED ARRAY ANTENNAS USING FIBER GRATING PRISM, IEEE Photon. Technol. Techn. Lett., Vol 9, pp. 241-243, 1997, a tunable delay line consists of a series of discrete fiber Bragg gratings having different periods. Each grating is designed to reflect a particular optical wavelength. The gratings are spaced a prescribed distance apart such that the required time-delay may be chosen by selecting the wavelength corresponding to the desired grating position. An antenna array may be fabricated by feeding each element with a custom delay line having a grating spacing proportional to the element position. The drawbacks of this scheme are that it requires many gratings, the beamsteering is discrete rather than continuous, the number of beam positions are very limited due to fiber grating limitations, and it requires accurate, precise spacing of the gratings in order to achieve time delays.
The serially fed discrete fiber grating beamformer is similar to the discrete fiber grating beamformer, but utilizes a single discrete grating delay line. See, Tsap et al., PHASED-ARRAY OPTICALLY CONTROLLED RECEIVER USING A SERIAL FEED; IEEE Photonics Techn. Lett.; PP. 267-269; Febuary 1998. The elements of the antenna array are controlled by serially gating the optical signal. This technique still suffers from the same drawbacks as the discrete fiber grating beamformer, and in addition, the types of microwave signals that can be handled is severely restricted.
A chirped fiber grating beamformer is an attractive alternative to overcome the problems associated with the discrete fiber grating beamformers set forth above. A continuously tunable delay line can be realized with a single chirped grating because the grating period varies continuously along the grating length. See, Cruz et al., CHIRPED FIBRE GRATINGS FOR PHASED-ARRAY ANTENNAS, Electron. Lett., Vol. 33, p. 545, 1997. A chirped grating beamformer in which every element is fed by a delay line having a chirped grating with a different length and chirp was proposed. See, Soref, FIBER GRATING PRISM FOR TRUE TIME DELAY BEAMSTEERING, Fiber and Integrated Optics, Vol. 15, pp. 325-333, 1996. Implementation of this beamformer for any practical array antenna is difficult for a number of reasons. First, because typical antennas require many nanoseconds of delay for proper steering, chirped fiber gratings with lengths in excess of 50 centimeters are needed. Such gratings have been demonstrated in a research environment but are not currently available. Also, this approach requires that the gratings be proportionally and precisely matched in length and chirp. Although this architecture has been proposed, it has not been demonstrated.
To circumvent these deficiencies, it was proposed and demonstrated to replace the long chirped fiber gratings in the system with identical, cascaded, chirped fiber gratings in a serial architecture where a single fiber grating might be common to numerous time-delay feeds in the system. See, Roman et al., TIME-STEERED ARRAY WITH A CHIRPED GRATING BEAMFORMER, Proc. 1997 Optical Fiber Comm. Conf., Dallas, Tex., February 16-21, Vol. 6, paper PD-28, pp. 479-482, 1997; Roman et al., entitled CHIRPED FIBER GRATING BEAMFORMER FOR PHASED ARRAY ANTENNAS, Ser. No. 09/058,352, filed Apr. 1, 1998. This design has some disadvantages which make it impractical in many applications. First, the design is not optically efficient in its current proposed form due to its serial architecture in which a portion of the signal from each feed is used to feed the next element in the array. Since array antennas nominally require a uniform amplitude in the feeds to the elements, all feeds in the serial architecture must be normalized to the smallest amplitude. Thus, while this approach minimizes the number of fiber gratings and optical circulators, it wasted optical power when standard 50% couplers are used, resulting in a low optical power at the photodetector. Low optical powers result in very poor microwave system performance rendering this approach useless for most applications. This may be partially remedied using custom proportional taps that are not commercially available. However, a straight forward analysis reveals that the tolerances required for such taps are not realizable, especially when they must be maintained over the wavelength tuning range. Second, the serial design is susceptible to single point failures. For instance, if the first fiber grating failed then all subsequent feeds would also fail.